The present invention is concerned with an apparatus for determining the condition of a pressure transmitting fluid, in particular, for checking or monitoring the water content or the boiling temperature of a hygroscopic brake fluid. Such apparatus typically comprises an electrically heatable sensor element immersed in the fluid to be tested, a power source supplying an electric current of constant magnitude to the sensor, and a measuring device measuring the voltage drop across the sensor element to determine the temperature-dependent resistance of the sensor element. The sensor element is of a configuration and size and the heating current being at a magnitude such that heating of the sensor element over the duration of the period of measurement causes a stable cellular convection to arise in a temperature range below the boiling temperature of the fluid. The voltage drop across the sensor element is an accurate indicator of the water content of the liquid.
An apparatus of the afore-described type is disclosed by German published patent application DE 35 22 774 A1. The sensor element of the apparatus is configured and the current supply devised such that at the time of measurement, a stable cellular convection arises. This requires a heating of the sensor element to a predetermined temperature which is within a temperature range just below the boiling temperature of the fluid to be tested, it being imperative for the sensor element not to be heated to the boiling temperature of the fluid under test.
According to the referenced German application, it is suggested to form the sensor element as a hollow body having an open wall; as a hollow helix; as a perforated tube or the like to favor the formation of convection cells and of the stable cellular convection, respectively, during the measuring time. However, sensor elements of that type are relatively costly which applies, in particular, if a stationary assembly in an automotive vehicle or in each of the wheel brakes of an automotive vehicle, respectively, is desired. Moreover, the requirements placed upon the mechanical stability of the sensor element, in that case, are particularly high.
The invention is, therefore, based on the problem of providing a mechanically stable yet low-cost sensor element for use with an apparatus of the afore-mentioned type, permitting the formation of a stable cellular convection during the measuring process.